1. Field of the Invention
The invention relates to a stimulated emission based optical detection system, and more specifically, to the optical detection system used for long-distance signal detection.
2. Description of Related Art
Various optical detection and imaging techniques are widely used for biomedical detection. Optical imaging techniques now used for biomedical imaging include fluorescence and nonlinear optics techniques. In nonlinear optical imaging, optical signals produced by the nonlinear optical interactions of photons and samples are detected and used to reconstruct images of samples.
Nonlinear optical techniques claim extended depth of field by using longer wavelength, which is useful when targets under biological tissues are difficult to detect because of the limited penetration of visible light. The technique is also effective for imaging the inner objects of light absorption materials.
Conventional methods of using optical signals for biomedical detection include polarization, absorption, fluorescence, Raman scattering, second-harmonic generation (SHG), third-harmonic generation (THG), and coherent anti-stokes Raman scattering (CARS), and other nonlinear optical signals. One example is fluorescence spectroscopy, which is widely used for measuring photoluminescence to detect changes in molecular energy. In the single-photon excitation mechanism, the fluorescent molecule emits long-wavelength photons after irradiation with short-wavelength photons. The emitted photons 110 are usually isotropic, as FIG. 1. Therefore, to improve detection efficiency, lenses with high numerical aperture which could provide broad solid angles are often used to collect fluorescent signals.
The above optical signal strength is usually very weak when compared to incidental light. To detect weak signals effectively, a highly sensitive optical detector or a high-intensity incident light source is needed. However, highly sensitive optical detectors are very expensive, and high-intensity incident light sources can easily damage samples. Another way to improve detection efficiency is to increase the lens aperture to enhance signal collection. Nonetheless, a high numerical aperture lens is usually limited by short its working distance. Overcoming the problem of detecting weak signals effectively at long detection distance is an important issue.